Dock levelers are commonly utilized to compensate for height differentials between the loading surface of a loading dock and the bed of a parked vehicle. In most instances, dock levelers are mounted within a recess or pit of the loading dock and are provided with a generally planar deck which is pivotally movable with respect to the loading surface of the loading dock. Dock levelers are also provided with a deck actuator which moves the deck between a plurality of operating positions including, for example, a raised position wherein a front header of the deck is inclined upwardly above the loading surface of the loading dock to facilitate the loading and unloading of vehicles having high beds, a cross-traffic position wherein the deck is substantially aligned with the loading surface of the loading dock, and a below dock level position wherein the front header of the deck is inclined downwardly beneath the loading surface of the loading dock to facilitate the loading and unloading of vehicles having low beds.
Basically, there are two categories of dock levelers: (1) biased-up or mechanical levelers; and (2) biased-down levelers. With biased-up levelers, one or more mechanical mechanisms, such as springs or other biasing members, are provided to bias the deck toward the raised position. Once in the raised position, the deck must then be walked down to a lowered position. A hold-down device, such as a ratchet and pawl arrangement, maintains the deck in this lowered position, against the biasing of the springs or other biasing members. With biased-down levelers, the deck is biased toward the lowered position by force of gravity. Thus, a powered actuator is required to raise the deck. Powered actuators may take the form of a hydraulic cylinder, an airbag, a linear actuator such as a screw-drive, or the like. With screw-drive actuators, a counter-balancing means is usually provided to make the deck easier to raise.
In order to span the gap which may exist between the deck of the dock leveler and the rear end of a parked vehicle and to provide a bridge between the loading dock and the vehicle, dock levelers are typically provided with an extension plate or lip which is hingedly attached to the front header of the deck. In operation, the lip moves between a pendant position wherein the lip is inclined downwardly with respect to the deck, and an extended position wherein the lip extends outwardly from the front header in a generally parallel manner with respect to the deck. In the extended position, the lip is adapted to rest upon and be supported by the bed of the parked vehicle and to provide a bridge between the loading dock and the vehicle.
There are a variety of mechanisms known in the art for actuating the lip of dock levelers. In hydraulic dock levelers, for example, an independent hydraulic cylinder from the one used to actuate the deck is employed to extend the lip. Alternatively, the powered upward or downward motion of the deck can also be utilized to extend the lip. In the case of using the upward motion of the deck to extend the lip, a snubbing cable is typically provided between a linkage carried on the deck and the frame for the dock leveler, as disclosed, for example, in U.S. Pat. No. 5,323,503 (Springer). The linkage, in turn, is connected to the lip. When the deck is raised upwardly toward the raised position, the snubbing cable is stretched, and the linkage is rotated to a position where the lip is extended. In the case of using the downward motion of the deck to extend the lip, a mechanical toggle mechanism is typically employed to extend the lip as the deck descends from the raised position by either force of gravity or by walking onto the deck, as disclosed, for example, in U.S. Pat. No. 5,475,888 (Massey). For this reason, such lips are commonly referred to as "walk-out lips".
When dock levelers are not in use, the deck is normally positioned in the cross-traffic position and the lip is normally positioned in the pendant position. In order to support the lip during periods of inactivity, dock levelers are typically provided with a lip keeper which receives and retains the lip when the deck is in the cross-traffic position and the lip is in the pendant position.
Before loading or unloading a parked vehicle, the deck is normally pivoted upwardly a sufficient amount so that the lip may exit the lip keeper and swing outwardly from the pendant position without contacting the rear end of the vehicle. Once the lip has cleared the vehicle and is in the extended position, the deck and the extended lip are then pivoted downwardly as a unit until the lip rests upon the bed of the parked vehicle and spans the gap between the deck of the dock leveler and the rear end of the vehicle. When the gap is eliminated in this manner, the vehicle may be safely loaded or unloaded with cargo, freight, or other material.
Dock levelers are also often used without the lip extended in so-called end-load situations. When a vehicle arrives at the loading dock, the cargo therein may extend all the way to the rear edge of the vehicle. Accordingly, there is no room on the bed of the vehicle for a conventional, extended lip. Similarly, when loading a trailer to full capacity, the last loads placed on the trailer will need to be positioned where the extended lip normally would rest. For both of these situations (referred to herein as "end-loads" or "end-loading") the dock leveler needs to be used without the lip being extended, i.e., remaining in or near its pendant position. If the trailer bed is either at or above dock level, the placement or removal of such end-loads can typically be done with the deck in the cross-traffic position. The forks of the forktruck are simply extended across the gap between the deck of the dock leveler and the rear end of the vehicle for the purpose of placing or removing the load. If the vehicle bed is above dock height, the forks can simply be raised up to that level, a feature provided on most forktrucks. If, however, the bed of a vehicle on which an end-load is going to be placed, or from which an end-load is going to be taken, is below dock level, the deck must be lowered to a below dock level position. Of course, the deck must be moved to this below dock level position without the lip being extended. Thus, when moving the deck from the cross-traffic position (with the lip stored in the lip keepers), the deck must first be raised to allow the lip to come out of the keepers, and then the lip must be partially extended so that it can avoid the lip keeper as the deck is subsequently rotated or lowered downwardly toward the below dock level position. Of course, the lip cannot extend a substantial distance, or else it would catch on the bed of the vehicle and interfere with the subsequent endloading operation. Thus, it is desirable in such a situation to have a mechanism that can extend the lip partially, so that it will miss the lip keeper as the deck moves downwardly.
In levelers with separate hydraulic actuation of the lip and the deck, partial lip extension can be achieved by actuating the lip cylinder momentarily. In mechanical (i.e., biased-up) levelers, or other levelers in which upward or downward deck motion is used for lip actuation, present means for achieving such partial lip movement is a push-bar mechanism, actuated by a manually-operated chain. The pulling of the chain by an operator causes a push-bar to engage against the back of the lip in order to rotate it slightly. While some mechanical advantage can be gained by utilizing a linkage between the pull-chain and the push-bar, a substantial force must nevertheless be exerted on the pull-chain to move the lip even this slight amount. A lip typically weighs approximately three-hundred (300) pounds and the necessary pull force on the pull-chain is thus approximately eighty (80) pounds. Of course, this force has to be maintained on the pull-chain until the tip of the lip has passed the lip keeper as the deck rotates down. In addition to requiring a significant pull force, the present pull-chain/push-bar arrangement is also expensive as a substantial push-bar and pull-chain, as well as a rugged linkage between the two, needs to be employed for the purpose of what typically amounts to only occasional need to move the deck to a below dock level position for end-loading.